When reaction products can be separated from one another or from the reactants, such as by distilling off a volatile product, the chemical equilibrium composition can be continuously shifted to obtain virtually complete conversion in otherwise equilibrium-limited reactions. If a chemical reaction is carried out in a chromatographic column, reaction and separation can occur simultaneously. The separation that occurs when chemical reactions are carried out in chromatographic columns can be exploited to increase reactant conversions beyond the thermodynamic equilibrium limit that exists in well-mixed reactors. The separation permits reaction products of high purity to be obtained, and if the reaction is equilibrium limited, conversions that are significantly greater than the maximum attainable when the reaction is carried out in the absence of separation may be achieved. For example, separation of the products B and C from one another in a reaction of the type A{character pullout}B+C prevents the reverse reaction, so that A may be entirely converted into B and C.
The low throughput that inevitably results from periodic injection of reactants severely limits the use of conventional reaction chromatography for practical chemical processes. Scale-up of these essentially batch processes must be done by increasing the column diameter. Large diameter columns, however, often lead to poor peak resolution due to column packing nonuniformities. Although this problem has been overcome in some instances for use in production scale gas chromatography, the technique is not generally available.
The problem of low throughput has been addressed by the use of a continuous chromatographic reactor, which permits a reaction to be carried out continuously with the separation of individual chemical species. This has been done with systems, such as those having a rotating cylindrical annulus or a countercurrent moving bed, which permit relative motion between the chromatographic bed and the reactant inlet. The simultaneous reaction and separation of reactants and products in such a single reactor-separator on a continuous basis can provide high purity products, and may shift equilibria to increase the yield of products. Recent research on continuous countercurrent moving bed chromatographic reactors (CMCRs) has shown that conditions may be found where product purities in excess of 99%, and nearly unit conversion of equilibrium limited reactions may be achieved (Cho et al., Proc. R. Soc. Lond., A283, 147-189 (1982); Petroulas et al., Chem. Eng. Sci., 40, 2333-2340 (1985)).
These features make CMCRs attractive candidates for chemical processing. Continuous reaction/separation systems may have economic advantages over more conventional methods, not only because conversions may be enhanced, but because the chromatographic separation will do away with, or at least decrease both the capital and energy costs of separating products and reactants. These process aspects are expected to become increasingly important factors in view of increasing emphasis on regulatory compliance and environmental considerations, which create a need for process improvements to enhance competitiveness.
In a CMCR, granular solids flow slowly past a feedport, against a counterpropagating flow of an inert carrier. A number of problems associated with the solids movement may be experienced, however. A solids handling system for recycling solids must be included in a CMCR system. Problems with solids breakdown and attrition, with the resulting requirement for removal of fines, are often observed. The maintenance of a uniform solid flow may also be a significant problem, particularly in large scale operations using large diameter columns.
The process aspects of a countercurrent moving bed can be simulated by successively switching feed and product take-off streams through a series of inlets located at intervals along a fixed bed or series of compartments. The shift of these positions in the direction of the fluid phase flow simulates movement of solids in the opposite direction. In this way the problems associated with solids flow can be avoided. This type of process (Sorbex) has been very successfully developed by Universal Oil Products (Des Plaines, Ill.) for the separation of binary mixtures. A simulated countercurrent moving bed chromatographic system, however, has not been used to carry out combined reaction-separation operations. Further, studies of simulated countercurrent moving bed chromatographic systems have focussed on liquid-solid systems and very little research has been carried out on gas-solid systems.